CM_1977_E_55.pdf (3.500Mb)

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International Council for the Exploration of the Sea

C. M. 1977/E:55

Fisheries Improvement Committee Ref.: Shellfish and Benthos Ctte

2.

3.

4.

5,

6,

7.

8.

9.

10.

11.

THE EKOFISK BRAVO BLOW OUT Compiled Norwegian . Contributions

Introduction and preliminary findings.

by Grim Ber ge

The physical environment and drift of oil.

by Rikard LjiiSen.

Determination of petroleum hydrocarbons in the water.

by O. Grahl-Nielsen, K. Westrheim and S. Wilhelmsen.

Fate of the flo.ating oil.

by O. Grahl-Nielsen, K. Westrheim and S. Wilhelmsen.

Occurrence and distribution of particulate oil following the Bravo blow-out.

by Thor Heyerdahl jr.

Oil-degrading bacteria and fungi.

by Steinar Pedersen.

Microbial counts in the Ekofisk area of the North Sea.

by Geir IndrebiiS and Ian Dundas,

Phytoplankton and primary production investigations.

by Francisco Rey, Kjell Seglem and Magnus Johannessen, Net- and nanoplankton: Effects of the Bravo oil spill.

by C. Lannergren.

Zooplankton, fish eggs and larvae.

by H. BjiiSrke, E. Ellingsen and S. A. Iversen,

A note on observations of fish in the area of Ekofisk Bravo.

by John Lahn-Johannessen, Odd Smestad and Erling Bakken.

- 1. 1 -

Introduction an~reliminary findings

by

Grim Berge

Institute of Marine Research, Bergen Norway

The preliminary re sults from the investigations carried out during and immediately following the Ekofisk Bravo blow-out

were published in a previous report (Institute of Marine Research, 1977). The report had a limited distribution, therefore its major parts are included and updated with further results in the present papers. The field observations were completed by the end of

July. Considerable material, however, still remains for analytical treatment and these papers are therefore also to be considered

"preliminary".

The blow-out occurred on Friday night April 22 at the oil pro- duction platform Bravo, situated at 56°33'N, 03⁰12. 2 'E within the Ekofisk field in the North Sea. The blow-out resulted in discharge of crude oil and gas in a mixture of 2: 1 through an open production pipe about 20 m above the sea surface. The dis- charge rate was at the time of the accident estimated to be 3 - 4000 tons of oil per day, (loc. sit. 1977). Later evidence of the presence of obstructing items in the pipe suggests that this was an over-estimate, and a more probable rate would be 2 - 3000 tons of oil per day. The mixture had a temperature of more than 75⁰C at the escape point, and it was blown another 30 m into the air where it partly dispersed and evaporated before the remainder showered down over the sea surface. Depending on wind and surface water movements, the resulting oil spread in different directions and attained varying shapes and consistencies, patches of oil up to 1 cm thick and water-in-oil emulsions interchanging with the more commonly occurring thin film and streamers of oil.

- 1.

2 -

The blow-out lasted for

7i

days until, after several attempts, a team of experts succeeded in capping the well at 1105 on April 30. It is assumed that about 40% of the escaped oil had

by then evaporated, indicating that some 9-13000 tons still remained on the sea. This figure is only preliminary, as an official

commission is still investigating the accident.

In order to appraise the dimensions of the Bravo accident, a brief review of some previous oil disasters that caused world- wide publicity may be useful:

The "Torrey Canyon" grounding, 1967 118 000 tons Scilly Isle s

The Santa Barbara blow-out 1969 15 000 ¹¹ The Chevron, Gulf of Mexico blow-out 1970 5 000 _"

The "Metula" grounding

Straits of Magellan 1974 50 000 ¹¹

The "U r Quiola" grounding

La Coruna, Spain 1976 100 000 ¹¹

The "Argo Merchant" grounding

Nantucket Island 1976 26 000 ¹¹

Viewed in this perspective of recent major oil spills, the Bravo blow-out may seem moderate. However, it all happened in an area of considerable fishing interest and without the success in capping the pipe it might have ended up as a major disaster.

Ekofisk crude oil is of low viscosity and under the described circumstances it spread rapidly over the surface of the sea.

It was consequently difficult to recover by mechanical means.

The oil had a high content of volatile aromatic hydrocarbons which are also known to include the more toxic compounds to marine living resources. It is easily dispersable into water by chemical means. However, chemical dispersion was assumed to increase the hazard to sensitive components of the living resources, espe- cially drifting eggs and larvae. Since these were most susceptible and no coastal interests seemed to be immediately threatened, it was decided to leave the oil drifting meanwhile, keeping this decision open for reconsideration if the situation should change.

- 1.

3 -

It was additionally decided to apply all available mechanical means to recover as much of the drifting oil as possible, and 800 - 1000 tons were therby recovered. Chemical dispersants were only applied for safety reasons on one occasion when about 50 tons were sprayed onto oil drifting toward·s the inhabited Ekofisk City.

The North Sea contains fish resources of considerable importance to the coastal countries. International fisheries in this area amount to about 3 mill. tons per year, as exemplified by catches of the major species in 1975:

Saithe 271 148 tons Herring 365 209 ¹¹ Mackerel 317 8CO ¹¹ Norway pout 559 600 ¹¹ Sand eel 424 800 ¹¹

Cod 219 976 ¹¹

Haddock 190 118 ¹¹ Whiting 168 099 ¹¹ Plaice 124 193 ¹¹

Sole 18 761 ¹¹

Blue whiting 41 000 ¹¹

These resources reproduce at rather specific spawning sites spread over the entire North Sea, some of which are located around the Ekofisk field. This applies, for example, to the very important mackerel resource, where Ekofisk is centered in the middle of its spawning area (Fig. 10. 6).

Several of the fish are spring spawners, where spawning is more or less timed to the vernal development in the plankton. During winter and early spring poor stratification in the waters and the lack of light result in poor plankton production. Stratification in the central North Sea will normally develop in April, and a typical spring bloom of phytoplankton then begins. It is followed by

spawning and rapid developments in the zooplankton. Eggs and nauplii of copepods, especially Calanus spp~, constitute the most

- 1.

4 -

important food for the fish larvae, and their abundance is a pre- requisite for success in fish reproduction.

It was feared that the blow-out might coincide with this sensitive stage in the biological development of the fish, and especially that products of the mackerel spawning might be exposed to the oil and its effects.

It was recognized to be the task of this Institute to describe the exposed living resources and the possible effects of the oil on

their physiology and behaviour as well as the distribution and fate of the oil in the marine environment. An improvised program was developed containing the following elements:

1. The occurrence and distribution of living resources, with emphasis on plankton, including fish eggs and larvae.

2. Recording of ongoing fisheries in the area threatened by the oil spill and the abundance of fish resources on which they were based.

3. Recording of possible irregularities in mortality and development of plankton and fish larvae.

4. Sampling of sea water for chemical analysis of the horizontal and vertical distribution of petroleum hydrocarbons.

5. Sampling of fish and plankton for chemical analysis on contents of petroleum hydrocarbons.

6. Sampling of oil from the surface for chemical analysis of the environmental effects on the oil.

7. Distribution of oil degrading micro-organisms and the effect of oil on the micro£lora.

8. Experimental studies on acute lethal and sublethal effects of the contaminated waters on larvae of fish and of invertebrates.

- 1.

5 -

9. Occurrence and distribution of particulate oil following the blow- out. Recording of oil drift.

10. Standard hydrographical program to describe the physical environmental condition.

The program did not cover all possible aspects of the encountered pollution situation. It was, however, realistic in relation to

available experts and equipment within our own and related insti- tutes, and could be put into operation at short notice. Recognizing the international interests in the pollution effects and also with a view to obtaining a broader expertise, invitations to participate were given to fisheries scientists of other North Sea countries and

to ~xperts from other Norwegian laboratories.

With certain adaptions to the continuously changing situation, the above program constituted the basis for a sequence of observations, some of which covered the entire period from 36 hrs after the outbreak until 2 months after its closure (Table 1. 1)

Table

1.1.

Ships and survey periods

Ship Date

R/V "Johan Hjort March 8

-

³¹

KNM "Sleipner" Aptil 24

-

²⁵

R/V "G. O. Sars" April 27

-

^{May 1}

R/V "Johan Hjort" April 27

-

^{May 1}

R/V "Johan Hjort" May 1 - 4 R/V "G. O. Sars" May 10

-

¹⁶

R/V "Johan Hjort" May 31

-

^{June 17}

R/V "Johan Hjort" July 11

-

³⁰

Based on information on the distribution and drift of the oil, grid systems of stations were planned for each cruise to cover both the polluted waters as well as the neighbouring non-polluted waters, for reference (Fig. 1. 1 - 1. 7).

- 1. 6 -

In addition to the Norwegian research activities, several of the neighbouring countries offered their cooperation and executed

coordinated programs. R/V "Corella" from Lowestoft, R/V "Explorer"

from Aberdeen and R/V "Dana" from Copenhagen which were on the scene during or shortly after the blow-out also communicated their activities and findings to the Norwegian research vessels. This information was highly appreciated and enabled us fo carry out our additional task of twice daily reporting the developments in the field to the Norwegian au:thorities in a better way. Ger:man and Swedish

research vessels also carried out specific research programs, and Phillips Petroleum Co., the operator of the oil field concerned, organized their own research team. The value of these joint efforts is appreciated especially in re spect to certain intercalibration pur- poses, as well as for the discussion of the different approaches and experience gained in the complex problem of recording fate and effects.

The following findings are extracted from the present reports:

Variable winds in the period following the blow-out transported the resulting oil slick back and forth within a rather limited area.

During the first 2 - 3 weeks the transport was dominantly in a northerly direction, and then in the next 4 - 5 weeks it was in a southerly direction (Fig. 2. 15). Towards the end of June the

o o ' 0

patch was centered around 54 30'N and 2 30' - 3 E, (Fig. 5.3) and had by then drifted over the area between this position and 5S⁰30'N and 2⁰ and 4⁰E. At the end of July oil was observed near the Ekofisk field but chemical analyses identified this as a mixture of Ekofisk and other oils.

The occurrence and consistency of the oil changed with ti:me, the original patches being broken up and gradually attaining a granulated appearance. Fro:m the second week onwards only patches of granu- lated oil were observed. Three months after the blow-out a mini- :mum of 150 tons or 1. 710 of the assumed oil spill remained as drifting tar balls in the surface layers. An unknown amount

remained suspended in the water masses and/or sedimented out on the

- 1.

7 -

bottom. The tar balls were for the latter half of the elapsed period scattered over an area of at least 55 000 km 2, in concentrations in June/July averaging 2.5 mg/m2

sea surface, which has previously been classified as heavy pollution. Follow-up

investigations may reveal the lifetime of these tar balls in the North Sea. Oil degrading bacteria were present all over the area, but seemingly the fresh oil had some inhibiting effect,

thereby reducing the numbers of bacteria 'recorded in the surface water near the platform during the initial stages of the blow-out

(Fig.

6. 1 - 6.3).

Total counts of micro -organisms about

1

week later showed even distribution and their abundance did not indicate traceable effects of the oil.

A number of oil samples from the surface of the polluted area, collected either by bucket or by Otter trawl, were analysed chemically. The samples contained from 30 to 70% water.

The dis appearance of the lighter components of the oil due to weathering processes accounted for a loss of more than 50% of

the original weight after a few days. With reference to the relative composition of aromatic hydrocarbons with low volatility, it was possible to identify oil lumps collected in June and July as Bravo oil and to distinguish them from lumps of other origins.

Oil-in-water emulsion wa s detected in <:oncentrations of up to approximately 300

surface oil in

micrograms/l in water under relatively fresh the near surroundings of the Bravo platform.

Small but significant amounts of dis solved aromatic hydrocarbons were found over a larger area (Fig 3.2 - 3.5). No vertical gradient could be detected down to 10 m. Except perhaps for the area of very fresh oil pollution, the concentrations recorded in the water column were below those observed in laboratory tests as having acute lethal effects on the more sensitive stages of fish development.

Hydrographic observations indicated that there were various

distinct water masses in the Ekofisk area during and immediately after the blow-out. Dominating was a cold core of winter-fol-med

- 1. 8 -

water in the central part of the investigated area, which was covered by an clpper layer of coastal water to the north-east and to the south-west was limited by Atlantic water (Fig. 2. 2).

At the outbreak the temperature of the surface layers was below normal, and except for the north-easterly corner of the grid

(Fig. 2. 2) no stratification existed (Fig. 2. 14). From May on, stratification . developed gradually and the homogeneous top layer decreased from 50 m in April to less than 20 m in the middle of May.

The hydrographic situation was reflected in the biological develop- ment, which during and immediately after the blow-out on average could, be characterized as being in an early spring stage. The vertical mixing removed the producing stock of phytoplankton from the euphotic zone and resulted in slow progress and long duration of the primary production in step with the development of the transition layer.

The variations observed in the rates of primary production and the stock of chlorophyll were generally attributable to differences in the physical conditions of the respective water masses. Statistical treatment of the production indices howeve r, revealed that shortly after the outbreak an area limited to a few square nautical miles around and eastwards of the platform had significantly reduced productivity, thus indicating an oil effect (Table 8. 2).

The phytoplankton was dominated bY' larger types (diatoms), where the fraction larger than 30

pm

counted for more than 50% of the primary production (Table 9.

1).

No significant change in the

contribution of different size groups of ~the plankton in the primary production (Table 9.2), was observed under the varying exposures to the oil hydrocarbons. '

Similarly, as for the phytoplankton, and pos sibly as a consequence of the

delayed development of the' same, the zooplankton biomass during the shortly after the blow-out was low. Krill was most abundant, and

Calanus spp. were dominant among the copepods.

- 1. 9 -

The progressive development of the plankton seemed normal, and except for a smaller area near the Bravo platform, no obvious differences were observed in distribution and composition within and outside the polluted areas. Repeated observations, however, showed the presence of dead copepods near the platform, and together with the reduced primary production indices in this locality, this indicates acute lethal effects of the more freshly discharged oil.

There were few fish eggs and yolk sac larvae in the area during the outbreak and immediately following it, and those found were mainly those of long rough dab and the cod family (whiting, haddock and cod), occasionally mixed in with eggs of dab and plaice. A few sand eel larvae were found in the southern part of the region.

Mackerel spawning commenced about the middle of May and its further development followed a normal pattern. The ichtyplankton seemed healthy and no obvious effects of the oil were revealed on it.

There was low abundance of fish at the time of the blow-out and for a short time afterwards. Very few pelagic fish were recorded,

the main component being O-group herring scattered more or less o:ver the entire area. Demersal fish occurred in quantities

estimated to be approximately 0,5 tons/km2

on overall average, with the highest abundance to the north-east of Ekofisk (Figs. 11. 2 -11. 4).

This picture seemed to change little in the following months, except for pelagic fish where the abundance of mackerel

increased markedly. No obvious effects of oil were revealed in relation to fish distribution or abundance.

Not all the observations have yet been reported on, and important data on the contents of hydrocarbons in fish and plankton when analysed, will add valuable evidence as to pos s'ible effects.

On summarizing the findings so far, however, they all indicate that the acute effects Were small. Although sublethal effects cannot be excluded, the low concentrations of hydrocarbons in the water columns outside the immediate neighbourhood of the platform, combined with the scarce availability of sensitive resources, makes it unlikely that serious acute harm on the resources should result.

- 1.10 -

It is evident that several factors account for this: the high

temperature of the oil on escape, and the fact that winds and rapid surface spreading caused efficient evaporation of the most volatile and

toxic compounds. Furthermore, the unstable conditions of the water masses resulted in effective dilution of dispersed and dissolved hydrocarbons.

2' I' o·

61' _ _ '---_1-

[1 I'{i

8",

,jf'1"

~50

60~

~

Fig. 1. 1.

I' 2' 3'

"

^{S' 6' 7'}

CTD-stations, R/V IIJohan Hjortll March 8 - 31, 1977.

0' g' 10' 11' 12'

- 1.

11 -

03°00'

--~----_~I~----~----

)(

"

& 1

o

2

)( 3

56°20~' ____ ^~~~ ______ ^~ ____________________________________ ^~

Fig. 1.2. Station grid system for KNM "Sleipner", 1: Bravo platform, 2: Stations Sunday 24.4., 3: Stations Monday 25.4.

~,'1"30

S7'OO',

56'30'

- 1. 12 -

0)'00'

" I : . 03'30'

I

01. 'o~'

I 01. '30' 05 '00' 0')' 30'

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2W

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... : ... 0..*

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233

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^X

Fig, 1. 3,

hi

¹

---0- 2 .... x· .. ₃

+ 4

•

⁵

b 6

'l

⁷

---,---~---~

Station grid systems for R/V "G. O. Sars lt April 27 - May 1,1977 R/V "Johan Hjortl' April 27 - May 1 and May 1 - 4, 1977.

2: ItG. 0, Sars" stations, 3: 'IJohan Hjortlt stations, 4: Current meter moorings, 5: Telemetric buoys, 6: Bottom trawl stations, 7: Pelagic trawl stations.

- 1. 13 -

58°304' -~-~---:---t-~--~-

B B

58· _{247 Y}

243 o 1 V 2 B 3

57·30' B

2 9 253

B

57~·--.J~--~-&----~~~-~~---t2~5~5---r

B

56⁰304' ---l~4=----e-

__

r__--&---4~2;;;;66.---r

Fig. 1. 4. Station grid systems for R/V "G. O. Sars" May 10 - 16, 1977. L CTn - sonde, phyto- and zooplanktonstation, 2. Grab stations, 3, Biotest stations.

- 1. 14 -

,,'

".

\

crD Sf.

Fig. 1.5. Station grid systems for R/V tlJohan Hjorttl May 31 June 17, 1977.

- 1.

15 -

2' 0' 2' 4' 6'

"

61'

f ^C

60'

.,

~

, .

^{100 99}

59' I~~ ^~

101 102

58'

104

57'

~

^{108 107}

55'

Fig.!.

6.

TRAWL ST.99-110 <!> 146 PLANKTON ST.

Zooplankton stations for R/V "Johan Hjort'I,

~ay 31 - June 17, 1977.

I '

-1.16 -

61o~2~0 ____ 1~0 ____ OL· ____ ~ __ ~ ____ J -_ _ ~~LO _ _ -.~ _ _ - l ____ ~7_0 __ ~8_0 __ ~L-__ ~10·

60'

59⁰

A

z735 5S⁰

57⁰

751 z .

805./.z

Z

/ . 75:

zZ z ·

Z/~

z' I~ '1

z/ 753z z - z

. . \

783 Z-Z--2-:<\--Z-2-z-z- z-z-z-Z-Z ·z-z-z-z-z-z-z-z-z-z761

1 \ · . . I.

^{SOO t-}Z--z- z- z 804

785z' z-z

~z ~z

~ 787 z-z-z-z-z-z-z-:t:>-z ~z 795

/\1

/ z

z I z

i \

740~

56'

ST,NO,561-807 CTD

2° 1· O· ,. 2° 3° 4" 5· 6⁰ 7' 8· 10'

51°;---~~--~----+_--~ ____ ~ __ ~~ __ ~c_--~--~----~

50·

.59⁰

58°

57°_

56°

1> 63 PLANKTON ST. x 55 CHLOROPHYLL ST.

Fig. 1.7 a og b. Station grid systems for R/V "Johan Hjort"

July 11 - 2 9 , 1 9 7 7 .

-1.17-

{J

. ' #

~

^.~

60₀ . I 0

59⁰

5BO

57⁰

56⁰

c

r"-~-+-~-o--_+~+~+

..

^f-~-+-+-+

l~ ~-6~

• t' ...

~+..-~-+-+-+-~-+-++::-+

.31 OIL HYDROCARBONS ST. + 63 OTTER ST.

Fig. 1. 7. c. Station grid system for R/V IIJ oh an Hjort"

July 11 - 2 9 , 1 9 7 7 •

2.1

2. 'l'he physical ocean environment and drift of oil , by

Rikard LjS?5en

Institut~ of Marine Research, Bergen, Norway

METHODS'

The description of the hydro graphic situation is based on observations from six cruises during the period from the beginning of March. to the end of July 1977. The station grids from the cruise's are shown in

Figs.1.i,:1. 3,1.4, -1.5· and 1. 7A. Oil the cruise with R/V "Johan Hjort" from Apr-il, 2·7 to Ma.y' 5 Nansen casts were 'made at standard depths to

obtain water samples for salinity determination and to measute temperature.

A CTD sonde' was' .used to record salinity and temperature versus depths on the other cruises .

'Recording, current meters were moored and drift bouys were put out to

m'l~asuJ.!e the wind drift (Fig. 1.3). The signals transmitted from the drift buoys were picked up by the NIMBUS

6

satellite in order to establish the po sitions of the buoys. The drift was recorded at the NASA center and sent to the Norwegian Meteorological Irtsti'tute, Oslo.'

The .occurrence of oil was regularly recorded by aircraft and ships up to Ma.y

2:0"

.and later ,more occasionally.

The Norwegian Meteorological Institute and the Continental Shelf Institute jointly ,with The Norwegian Veritas both separately and in cooperation used com-

puter programs .to simulate and predict the oil drift,based on wind and current observations. The sightings of drifting oil (patches, slicks, tar lumps etc. } were used' to recalibrate the prediction models.'

Reports on these activities will be given by the institutions mentioned.

2.2

RESULTS AND DISCUSSION

Fig. 2.1 shows the temperatur.e and salinity at, the sea surface in Ma,rch, which represents a typical winter situation. The water column was nearly homogeneous down to the bottom except at the Norwegian Rinne. The transition zone between Atlantic water and Norwegian Coastal water was narrow. The movement of Atlantic wate r towards Skagerrak was shown by a tongue of water of relatively high salinity and temperature following the western and southern edge of the Rinne. The temperature of the surface waters increased from bplov,' 3⁰C near the Norwegian coast to 6-r7⁰C in the Atlantic water.

"",', ,-,,';", ','

During and'irrtme,diately afte,r, the blow put ,the hyc!,rographic:condjtion in the area atoul1dEKOFISK we:;e

ch~racterized

by fou,r distinct water m.asses. A core of cold water was observed between the sea surface and the bottom in the central p3..rt of the inve stigated area. The core

' 0 ~

tpmp'.'rature \vas around 5

C

and the scdinity slightly below 34,80t, (Figs. 2.2,2.3 and 2.11). This core was probably locally .formed by w'i nte r cool in g. The observed vertical and horizontal temperature and salinity condition indicated the presence of an eddy. The charaqter of the water ma.sses towards the south-western corner of the grid approached ,~B-at:,of Nor,th Atlantic water e. g. high, salinity and high ternperature(Figs'\ 2. 2 and 2,3). In the northern area, a subsurface wa,ter body waS found which, according to t-S analysis, must have ha cl a origin different from the other two water ma.sses. Above this water body, especially, in the eastern area, a 10 In thick typical coastal water layer was found with salinity well below 34,00/00and temperature above

6°c

{Fig. 2.2}.

The water masses were nearly homogeneous vertically, except for in the northern p3rt of the investigated area. The sea surface temperature was approximately 1o

'e

below the mean value for this timt~ of' year.

The ma.l11 fe'atuf'es of the hydro graphic situation did not change during the fi rst we-ek of Ma.y. The temperature of the cold core, however.

increased by approximately 0,4⁰ C (Figs. 2.4, Z. 5 and2. 11).

By the middle of May the coastal water masses had moved

further north (Fig. 2.6). The temperature of the upper layers, and to a lesser degree also of the bottom layers, had increased signifi-

2.3

cantly (Figs.2.6,2.7 and 2.11) .

. The hydrographic situC\.tion at the sea surface in the first part of June i sshs>wn .inFig. 2.8. J:he mean feature is that the Skagerrak waters flowed along the southern and western edge of the Norwegian Rinne dudng this pe:riod. These waters had .a relatively low salinity and

hightemperC\.t~r~ condition,. and their movements had been very complex, resulting in a convection layer 10-iSm thick off the south westerh coast .of Norway. ^I A zone <;>f upwelled water of relatively low temperature

was ,observed between the Skagerrak water and the Nor.wegian coast.

This. copditi9n was caused by a stable. northerly wind before and during. the tim.e of observations. Except for the south-eastern area the temperature s were well below the mean value for this part .of the month.

Corresponding main features occurred at the sea surface during the last part. of July (Fig. 2.9). The movement of the· Skagerrak waters, however, had a significant southe;rly COmponent and a bulk of Continental Coastal water had ITloved north~west, .Jointly the se two water ITlas se s covered the .south~eastern part of the investigated .area ..

Figs.2.10,2.11,2.12 and 2.13 show the vertical distribution of tempera- ture and salinity along the 57 N parallel and deITlonstrate the develop-o .rnent of a thermocline ~ The lateral movement of the low salinity c,oastC\.l ~aters is also ipdicated, Both these processes contributed to

" an in.C1~,easing vertical layering of the water masses in the. EKOFISK area which is deITlonstrated in Fig.2.14 . . Stability 103

{-z~L:-

^versus

tiITle of the transition layer between the upper and bottom homogeneous water .masses is plotted in the figur.e. The hOITlogeneity is defined as ..

the vertical variation of density being within 0,1

('t.

The abscissa to the left gives the C\.verage stability between. 3⁰ and 5⁰E. approxiITlately, along

the 57 o N, .paraUel.

Obviously the layering was insignificant until the middle .of Ma.y, and

th~n increased rapidly during June and July. The figure also shows the thickness of the upper homogeneous layer (abscissa to the right).

Vertical mixing had occLj.rred down to the bottoITl in March, but only . down to approxiITlatel y,17 ID. after the full development of the transition

layer in June-July.

2.4

The Atlantic water in the section was observed only as a subsurface bulk in June and July (Figs.2,12 and 2.13). The amount of this water mass also seemed to be small compared to that of the years 1968 -1973 at corresponding season and section (unpublished data).

Recordings from moored current meters from April 28 to June 14 stated that the semi-diurnal tide current was then domi.nant (ANON b 1977). A non-periodic component, obviously mainly wind-driven, was superimposed on this oscillation. Average speed of the current cal culated from all observations was 20 cm/ sec, and 10 cm/ sec. in the upper layers and at the bottom respectively. Up to Md.y 5 at the southernmost rnooring, the current at 10 m depth had a speed of 5, 5 m/sec. and was directed towards the north. Between May 5 and 13 the current had a velocity of 3 cm/ sec. and had an easterly direction.

From this date until June 6 the residual current again turned west and 0 btained an average speed of 10 cm/sec. during the last week of the period. On June 6 the current again Changed direction towards

tlH: cast, the speed being only approximately 2 cm/se c.

The recordings fronl the moored meters, together wlth current profiles from M,l..Y 13 and 14, demonstrate the existence of a significant

vertical current shear.

DriU_9_L~D_. Calculation of oil drift was mainly carried out at the Meteorological Institute in Oslo, using computer-based models. This activity has been preliminarily reported (ANON a 1977) and the com ..

puled data kindly put at the author's disposal.

The wind at the positions at which oil was observed was calculated e\ery third hour, based on observations from ships and from weather maps. In cases where no oil observations were available, calculated positions were used. Twenty-four hour averages of wind force and direction were used in order to eliminate tidal effects, and the pro-

bable Ekman drift was then computed. A wind stress factor of 3 and a deflection angle of 15⁰ to the right of the wind were introduced into the model. Residual current was not included. Whenever well defined oil patches were reported, the model was evaluated and the trajectories corrected. The agreement between the calculated drift,

2.5

based on the original stress factor and deflecting angle, and the observed '. ; ,drift of oil were ,as acceptable as could be expected from the uncertainty

" of the calculated wind, of observed floating oil, and 'disregarding Stokes velocity and pure, wind drift.

Fig. 2.

is

demonstrates the best approach to the theoretical drift of an

"approximate 'center" of the oH s licks" Fig. 2. i 6 shows the N ",.S and E":'W components of the wind. Evidently 'the N·S component of both

". wind and drift was dominant; . Up: to May i 2 the wind was southerly, and at the end of this period the oil reached its northernmost position.

From then on the wind was mainly from. the north and the oil drifted south,

~except for 'a short interval ,d£;time, June 6 to 15, when the wind and the oil drift oscillated in a NE-.SW' dhectiorL The calculated drift

seemed to correlate satisfactorily with'the findings of oil. on June 11-14 (Fig.5.3).

It seems evident that the oil drift, at least up to the end of June, was caused rnainly by the wind forces. This supports the hypotheses that the residual current at tbe sea surface is dDminantly wind driven in the central p3-rt of the No dh Sect (DOOLY 1974).

These specific wind conditions also contributed to the oil not approaching the coa stal current systerns and the shores.

/\ comprehensive experirnent with drift cards carried out in 1972 indicates that the opposite could easily have occurred a few weeks after the blowout (DONS 1977) ⁰

The drift buoys rnentioned previously rnoved with a son~ewhat higher speed than the oil (ANON a 197'7), Due to the simple method for recording the position of the buoy, however, this type of drifter may still be useful for first hand infonnation on oil drift.

2.6

REFERENCES

ANON

(B~rresen,

J. og Haland L.) 1977 a. Rapport om den

ekst~aordinrere

virksomhet pa Meteorologisk Institutt i forbindelse med utblasningsulykken pa EKOFISK B. Rapp. Oet norske

meteorologiske institutt, Oslo 1977:1-5. [Stens ]

ANON (AUDUNSON, T. et al.) 1977 b. Bravo utblasningen; felt-

observasjoner,

analyseresultate~

og beregninger tilknyttet oljen pa

sj~en. Rapp~

Institutt for

Kontinentalunders~kelser,

Trondheim. 1977 (90) (in press)

DONS, U. 1977. Beregning av

overflatestr~mmer

i

^Nordsj~en

i 1972 pa grunnlag av

drivkortfors~k.

Thesis (cand.real) ,

Universitetet, Oslo.

DOOLY, H.D. 1974. Hypotheses concerning the circulation of the

northern North Sea. J. Cons. Int. Explor. Mer, 36 (1) :54-61.

61° -

fift~ [J

, I ~h

60°

11

\

59⁰ 35

and salinity, sea surface,

57°30'

5'1'00'

- - - -^.. ---.-~...,....--.---.~

'~Q2 ---~

6.0

~

^{\ \}

//~J

/ /

~\ ^sV)

~~~s., 0 ) '

~~

^{'" 6.4 . 6 2 6.0}

---

^{. . 6.2}

^/

^6.4

^/

5 6 ' 0 0 ' - + - - - T - - - · - · - - · - - - . - - - , - - - · - - - · - - - 1 2°

57'30'

s %0

57'00'

56'30'

til

~

_35,0

34,8

56'00' - - j

2° 3° 4° 5° 6°

Fig. 2.2 Temperature, tOe, and salinity, S

%0,

at 5 m depth, April 27 - Mq.y 1.

- - - --- - - -

t o

c

57°30'

57°00'

56'JO'

56°00 ,---r--- ---,-- --- - - - ,-- - - -

2° J^O ~Q su

--- ---

---l

57°30'

I

57"0[)'

56°01)'-,--- ---,---

F'ig_, 2" 3

c;

⁰ o,:nd t,a 5 %q? at 30 rn depth,

..2 ,1

c,-·".1 .1'-

~~-~---.~----~-~--- ----.~--~---.~---.-

-. ---l

5.2

...

,

''\ "'.

(, I, 6.2

fiL

../'

~) 8 6(1

I /

6.1, /

.- --'1' -.--~.--.- -----r --- ---.. ---.. -

---___ 3~.8 ^--~.

.. ----.--.--~"."'"

\

\\

I

'i(\'OO - ~-~---.... ,-~---'~-~----·---T---.. ---~- --.----~

" 0

Fig. 2.4 T empera ure, t t,^OC, and salini ty,

S

%~, at 5 m depth, May 1 - 4.

60 52

)

57°00'"

56" 30',

5 b ' 0 0 ' , . - + - - - . -

--,---.---.--~---~---~

2⁰

s %0

31..8

---.---·---.I·~'=-~---~

Fig. 2.5 Temperature, May 1 - 4.

and salinity, S

%0,

at 30 m depth,

'"It,;;::fll ~---

5&'00'·

6,6 _ _ _ _ _ _

~~8

&,6

- - - 6 , 8

/7.0.

;,.11 i

S

%0

I',H':) "'.

~) :.'0 1 'j'.

57"00'

5fYJO'

Fig. 2.6 Ternperatu re, tOe, and saHni ty! S %0, at 5 m depth, May 11 - 14.

5~)O~·~--~---~----~~---~---~

58'00

~

---6,2

:no 6,0

,6,2

~'8

'J . ~,' '.

6,0

4 ') 6'

Fig. 2.7 Temperature, t o C, at 30 m depth, May 11 - 14.

5,'

60'

I

/

6"

57~

SS'

0' "'!'

. t-- -.L...---...I_----I. ___ • +--

I.'~~'~ ^/1

~

^P

^!Jl0

:l '

<.;

9

.

,\

I

9

I I I

\

\

\

10

?~~

35 34 33

$ %0

Fig. 2.8 Temperature, tOe, and salinity, S

%0,

at the sea surface, M3..Y 31 - June 17.

2'

60'

59'

I r ~~.

r . ,

)

~7~, ,

.!

i i I

t'

i

,~"

If

5~·1

L~\.

0' t' 2' 3' 8' q'

13 14 14.5

~

s ^0/00

Fig. 2.9 Temperature. tOe, and salinity, S '~/OO, at the sea surface, July 11 - 29.

'"

.... >- ~ z z :r t w o

'f so~

I

~

. ~-.-.

.~

! 1eo . .1

0,

r." 1" 1.. .•

,c

1.0 so 6° TOE L_

fn-

I --L-

! \5

6 5.5 5

\/ ..--- -_.---_

.. _-

---

--'

....-/

./

---~

// tOe , ' ,'J

1 __ . -_ .. _-,

r~L_----_---.-L __ ._._. ___ . __ ...1.-_. __ L __ .. ___ _. \ 35 S <35

1-;"" 'r' E ._. __ ._ .. L ._

,.,.--- ---_.

-,'

1 ,,1 ~ r---...''--- \ ~.-~---. 'OO~ ~-

/' --

~

. //

--

---/

s %0

Fig. 2.10 Temperature, tOe, and salinity, S 'JIor, along the 570 N parallel, Ma.rch 18 -19.

...

--..._-

o

./

5.9

/ 5.6 5.4

-

- - - / / _/

/

~-

---

^{/ /}

50

APRIL 27 - MAY 1 s: ^3~.7? - 34.94 %.

'3° 4° 5° 6°E:

0

6.0 \ 6.2

,Cl:

tOe /:.----' 5.6 6.0

'l1J 5.8

I-

~- /

;/::" \~

l1J ^--~ / '

~ ~

,~ ^{:r; 50} - - - 5 . 6 ~

1/5~\

!i: l1J

'"

, 0

MAY 1 - 4 S: 34.76 - 34.94 %.

o

50

MAY 12 - 13 S: 34.65- 34.83%.

Fig. 2.11 Temperature, tOe, along the 57° N parallel, April 27 - Md.Y 13. Salinity variation indicated

by num ':>er s.

If) er: lJ .. :z ~ :r f- a.. tU o 0') 'l°E

JT--

I

r ~--~ ~~- 8-=~

(,/"/ ~ ---,:::-,~~-~--:--

1, ,--~ !

~.. f _, _, . 6

1 !

---1.. _______ ... _ --'-. ____ i ___ \--_. ---J ... _ .. -\~--!.

J

!

"'"=tr ________

10____ '-' ~ _______

--~ ~~~~~ .•• ~~_~~5:~~ __ .. _=====:/~

----. -"-~

---

-._-

i ' --

"0--.-... '---'---,-/ ,

~ "--- - .--- -- -~

./ I -'- ~.J;-J . 1

/ ----./ tOe u' 10 :~ . .;. 5" c· 7° E 1 _ _ __________ 1." ______ <.-. ___ l ______ _ \ -~ 34 33_

G -r---~~.-__________ 1... ___ . __ _ J __ _

\~331~J--

5 -. 35

~---- --

.----_.----

-

--~ ---

/- / ..-

,_ __ 1

.. .J -~

./

s 0/00

Fig. 2.12 Temperature, tOe, and salinity, SF"o along the 57° N parallel, June 10 -12;.

---- -

-

--- -

III -",

...

tu 7 ~ I f- 0... w o

,~~~ o~ __

,,,

.l" 1.0 5° &0 7°E a

'\->: ______ ;;;;

.1

ll.J

'11.=J 11. \

~

;

--_.,,- .---.~~... ~~

.

. -C,~~~~-~C=-:::::::===~ .. ~~. ~ S--r

------.-------7

-=- _ I r- C~ J--- - .- .j r 1

...

---... //

' ...

----'

FJ'J 1 0'" 1° 2 fi-r--'-"

1 _____ -

~

---35

/

.

~ ··'11 ~ i-~I . / i ~~---~

'~r:; ~ Fig. 2.13 o Temperature, t C, er -,-':' .. ~ .~. ~" • and .. salrini ty, --.

.-

.1~

TL \

'-...

tOe , .. 0 :," ':JU 7°E L-__ \_'. ____ ...L.. __ _

\._ C ..

I ____ _______ 32 ' _--_...l.--

__ 31---7 -3t.~~

. ~/'~

---'- / ---~,/

---~

---- .-'

S

%0

S '1100, o along the 57 N parallel, June 23 -25.

200

150

bl

^N

'lJ 'lJ 100

50

Fig.

X, ...

, B

... , ...

'

^... _{... "\}

\

\

\

l<

\

MARCH

2.14

APRIL

A: stability, mixed layer.

\

\

\

\

\

\

\

\

\

\

\ x,

MAY

dz

"

'1,1

o

... x- - - ... x

JUNE , JULY AUGUST

70

60

50

40

30

, 20

10

o

III W f-er w ::£

and B: thic).<ness of the upper

.. 58°

.JUNiO 1 55°

55°

JUNiO 20

I:<~ig. 2..15 Drift pf oil, April 23 ~ June 2.0.

I.Il ::€ ~ I I W W ;L In o z :;;

t2 nAND E SOMPON~NT 10 -,9 8 'i L. 2 o 2 -I. .f; -A -10 -12

~ ,

;\ It I \

\ \ h \ ,q

r

I \

I ~l~: {t i o ,){ \1' .

P.

~

po..,), ,\

~

'. I I " ' '\ q ! r-1 I b, ! ',\ P-

i

^{! , ,Pr'}

^'~,

fl

7.6 • I

\0 ' /

:'1',

I,

11::>· ' , /' 01'"

\

\ ~, ' .1 _\I \ I j , 6 Q. :

bJ

'0

6. I \

,

't'l

,p

I:f SAND W COMPONENT l. to 15 20 2:'> ----_.--MAY

~ \

\

~. ,~ ~ 1/\ !.

9-q

,

10

~( , i

\ ! I J ~~ i'i

,p

,

, I ,I

q : I \'b

9 I "

' \/ \., ~) ~.

& " :,11\\

1'1 f·.·.

" _'.' 9 0, . '~II I : : 1 I '0-I I -6., I

I

s

N

, ' I I

" I1 ,I

"

~ 10

R

\ 15 ---JUNE

AI

I ' )1,

~

I I , I

,.

, I ' , I " I Q I _ \ l -0 \I b 20 .2:l 30/1 5

I 9

, q

I it ~ \ I \ I \ 1:1 \ I '? , I ci 10 15 JULY

0.. '0 18 Fi,g_,

z.,

1 b '1orth alld south components _ ---east and v/est com?onents of v.-ind, ?vb.y 4 ~ July 1 B.

3 . .

DETERMINATION

OF

~ETROLEUM

HYDR!OCARBONS Il:'l' THE WATER

by O. Grahl-Nielsenx

, K. Westrheim and S. Wilhelmsen . Inf3titute of Marine Research, Bergen, Norway'

The blowout initiated a massive chemical investigat~on

of

:the dynamics of thespilledoi:l in' the el1vironment. Ofpdme importance was tht;! determiriationof the amount and type' 6f , petroleum hydrocarbons which entered

the

watercol·umn, and the 'distr'ibution of the hydrocal'bons both hod~ontal1yand Vie

rti.c

ally.

This investigation' was based' on chemical analy.sis' of water sampled at the majority of the stations on six different cruises.

EXPERIMENTAL

W~ter Sa.mpling. Water was sampled with 2.81 bottlesm:ounte.d

011

a

frame with lead weights at the bottom and 'suspendedfr.om a buoy,. The bottle was stoppered' when lowered and the $topper was removed by pulling a string, Great care was taken to avoid concentration from visible oil on thesurffj-ce. ' As the main emphasis was placed on the horizo~ltal distribution of petrQleum 'hydrocarbons.,

samples from I'm depth were taken at mo~t stations. Atce~tain

selected stations samples were also collected from 5 and 10 rn to investigate the vertical distribution. In thtise yases a half inch hose was used for support of the water' sampler with the' string

for release of the stopper' inside the hose.' This rat4er primitive sampling' method worked well down to 10 rn, The bottles were

retrieved open, and the upper 50 - 100 mIoi water were immediately dfscharded.

Extraction'. During th~ "S1eipner" and the two IIG. O. Sars^il cruises the water was extracted immediately. The samples from th~ other

XTo whom correspondence should be aqdressed.

,-i"'.

- 3.2 -

c ruiseshad 30:mJ, c0l11m:n-:,,~istille.ddichloromethane. added to prevent biological activity, and' they- were then stored in the dark for subsequent extraction upon return to Bergen.

For extraction, the, wSiter ,sFtmple, was transferred to a 3 1 separatory

• 0 - ; , •

funnel with C3: teflo.n stopsock and stopper. ' The sample bottle was rinsed very carefully with 50 ml dichloromethane which was then thereafter also added to the separatory funnel for extraction.

This was performed _{, . ' .} by tl1orough hand shaking for 1 minute, After

;, , ~.., . ' , , ' . "

separat~on ,of tJ:J.e ,.?ichloromethC3:ne the extraction was repeated twice

-, " i . , . ;" " .

with ~5 ml each time, Both times the dichloromethane was first

1 q.sed f.p.r rill,s~ng of the sample bottle, After these rinsings the sa,mple bottle was ready fo;!:' the, next s,a.mpling. The combined

'I " , " ,I I \ "

extract,s compFised 60 T 70, ml due to slight sol\lbility of dichloro- methane ip seawater .and to s9me eyaporation from. seawa,ter. They were stored in the dark for analysis onshore. Controls were taken daily by going thorough the procedure three times with 25 ml

dichloromethane as described, but without seawater. In this manner c,ol1tr.ol of,cqntaminatio:q. <luring the. e:fCtraction and analytical procedures

. " " " f ~ ' , . ' .

aEl _{. ,} weJI as. of the _.' slE~al1~iness of the. $ample bottles was obtained.

, ' , . ' . ; " ~., '. "

A. few <::ol(tro~s of the efficiency of the extraction were made by

repeate?- extr~ct~on, was. checked

by

addition of 1

JP1

methanol solution containin,g 31,.9 J..).g Ek?fis~ cr,ude oil ,to 2.8 1 of uncontaminated

se,awater i1")" a ,,$eparatory fU1).nel, resulting in a concentrC\.tion of

\ . ..- .. . ~:.", - ~ .

. . ~ 1. 36 rg!l. ,.Aft(3+ ,.tho~oughmixing with the water, extraction was carried out,. as de,sc:dbed above.

-' ^"

Analysis., . ,The . e~tracts w.ere dr~ed with approximately 5 g sodiurp.

" sulfate, wl1ichhad bee

Il.,

freed from hydrocarbon ,contamination by so,xhlet extraction .;with dichloromethane. _. Thereafter appropriate

"" '. "

i. ,aliquot~ of the extractEl we,re, co;ncentrated 011 a rqtary evaporator at 15 - 20⁰C under reduced pressure from a water aspirator. The evaporation was stopped when approximately 0.5 ml solvent was left, and t,his'''Yas,qlla,ntit~tively tr,an~~erred to a small vial with a conically shaped bQJ:torn.", Further :conce.ntrating was achieved with a stream of dry nitrogen gas.